He walked into the main square by the National Museum, poured petrol on himself and lit a match. As the flames tore through his clothes and began to sear his flesh, the 20-year-old student could not have anticipated the massive effect his action would have. What might have been written off as a futile gesture became a defining symbol for a generation. Jan Palach's self-immolation in Prague's Wenceslas Square was arguably the most dramatic suicide of the 20th century.

His death, 30 years ago yesterday, brought hundreds of thousands of people into the streets. A quiet student of philosophy with no history of political activism, Palach burned himself alive five months after Soviet tanks had rolled into Czechoslovakia to end a swelling reform movement.

But the main target of Palach's dramatic protest was not the Kremlin. He was upset by the way his compatriots seemed to have accepted the occupation without resistance. 'After the euphoria of 1968, people had become depressed and beaten down. Palach wanted to shake them up,' says Zuzana Bluh, who as a student leader helped to organise his funeral.

His act was modelled on public self-immolations by Buddhist monks in Saigon protesting at the Vietnam war. It was later copied by a wave of Indian students who set themselves alight in 1990 in protest at changes in quota systems for entry into university and the civil service. In Britain in 1993, Graham Bamford, a 48-year-old former haulage contractor, burned himself to death outside the House of Commons to protest at the horrors of Bosnia. Three years ago an Italian homosexual set himself alight on the steps of St Peter's to denounce discrimination against gays.

None of those other suicides had the impact of Palach's. So what is it that makes one self-immolation catch the imagination while others do not? It is certainly the starkest form of suicide. Hunger strikes are often used as a form of escalating pressure by protesters, because their effect is slow and reversible. Suicide bombings are the ultimate political act, but as they are designed to inflict pain on others they carry different moral overtones.

Self-immolation is violent and non-violent at the same time. But what really distinguishes it is that it is one of the most painful ways to die. Palach was in excruciating agony for three days before he succumbed to his 85 per cent burns.

His act was powerful because 'it struck a raw nerve when society was going into a kind of oblivion about our situation', says Zdenka Gabalova, who is now a counsellor at the Czech embassy in London. In the note he carried, Palach wrote that he wanted 'to wake the people of our country up'. In the immediate aftermath of the invasion, workers at several Prague factories went on strike. The Communist party which had led the reform process known as the Prague Spring denounced the Russians even while Alexander Dubcek and the other leaders were taken to Moscow. But when they came back after signing the so-called Moscow Protocols, which attempted to legalise the presence of Soviet troops in Czechoslovakia, most people began to give up.

Palach wanted to reverse this pessimistic trend. He wrote an explicitly political note, calling for an immediate end to censorship and the banning of a Soviet propaganda sheet called Zpravy. He also implied that he was part of a group of volunteers ready to kill themselves in sequence if their demands were not fulfilled. Ominously, he signed his suicide note 'Torch Number One'.

It was a fantasy, as Zuzana Bluh recalls. One of her student union colleagues talked to Palach as he lay dying. 'He said he acted on his own. There was no group. But he insisted he had no regrets about his action, and didn't want anyone else to follow his example.' While he lay in hospital, negotiations started between the students and the government. Once Palach died, they narrowed down to the terms of his funeral. For several days his body lay in state in the hall of Charles University, placed deliberately near the statue of the religious reformer Jan Hus, burned at the stake in 1415 for asking for the Bible to be read in church in Czech. Meanwhile, thousands of Czechs kept vigil at the spot where Palach set himself alight. On the day of the funeral, half a million people took to the streets in by far the biggest demonstration since the Soviet invasion.

It was very different from the reaction to the self-immolation by Buddhist monks in Vietnam in 1963. The first suicide was caught on film and the shocking picture of the human pyre became as famous an image as the photograph of the little girl running down a road after an American napalm attack. But the suicides led to no action. 'They failed. They were seeking to break the impasse in Vietnam, but were denounced by both sides,' says the Buddhist historian Stephen Batchelor.

Their self-immolation was criticised by people who supported the anti-war movement, such as Martin Luther King. Thich Nhat Hanh, a senior Buddhist monk who was close to those who set themselves alight, said in a letter to King that their suicides were neither acts of despair nor political protest. They were aimed at 'moving the hearts of the oppressors and calling the attention of the world to the suffering of the Vietnamese... To say something while experiencing this kind of pain is to say it with the utmost sincerity.' Although Buddha was against self-mortification, various forms of self-immolation have been practised over the centuries in the Mahayana school of Buddhism, says Batchelor. 'This wasn't a one-off non-traditional reaction to an extreme situation.' That could hardly be said of Graham Bamford's 1993 suicide by fire at Westminster. Although unique in our recent history, hardly any British newspapers covered it in depth. As Bamford was known to be depressed over the failure of his marriage, the political aspect was played down. But in a letter offering his services to an aid agency shortly before his death, Bamford wrote that 'a photograph in the newspaper of a distressed little Balkan girl about the age of my own daughter galvanised me into action'. A few days later he wrote a final message: 'The British army must not be a guard of honour at a mass funeral. Bosnian babies, children, and womenfolk are waiting for the politicians to do what they know they should - give them military protection.' No British television producers felt the death was worthy of interest, and it was left to a Croat, Nenad Puhovski, to produce a film on Bamford which mirrored his own concerns about public apathy towards the horrors going on in the Balkans.

Psychiatrists have had little chance to study self-immolators. One of the few who has is Dr Swaran Singh, now at Nottingham University Hospital, who looked at what he calls the 'altruistic suicides' of 22 young people, 12 male and 10 female, in India in 1990. Nine set themselves alight and the others took poison, in protest at the government's decision to enlarge the quotas for low-caste people in universities and public service jobs. They were interviewed within two days of their action. Six of them later died.

Dr Singh and his colleagues found that, apart from one man, the young people had no manifest psychiatric disorders. They also contrasted with young people who commit suicide for non-political reasons. These suicides usually have a 'strong external locus of control', meaning that they believe in outside forces such as luck, fate and chance. Seventeen of the 21 had high aspirations and ambitions.

A similar analysis may have fitted Jan Palach. He had no history of depression. Ironically, his death had a kind of therapeutic effect on Czech society. For a few days the post-invasion trauma was lifted, as people poured into the streets in a collective act of solidarity echoing the demonstrations of the Prague Spring. His action seemed to have given them back a sense of dignity and honour.

When I visited his grave in Prague's municipal cemetery of Olsany on an ordinary weekday more than a year after his death, there were still hundreds of flickering candles. Every few minutes someone new came to lay flowers. So popular was the shrine that in 1973 the secret police exhumed Palach's remains and cremated them. His family had no idea where the ashes were until they were handed over in 1974 for burial in his local town. But, in a typically low-key Czech act of silent defiance, people continued to treat the empty grave in Olsany as a shrine. The piles of flowers were constantly renewed, even though Jan Palach's body had long gone.